I've read a few of the posts here, and I think there are a couple of things that people don't quite get yet.
Firstly, yes, Valve's "official" steambox will be running linux, and that might be seen as hampering adoption of the service, BUT:
There will be third party "steam capable" type machines being put out by various companies, the "steambox" you have on your desktop is perfectly capable of running this service, and the games that are/will be involved. Also, the linux version of Steam out now is just a beta. There aren't all that many linux games yet, but hopefully that will change.
I have my own desktop PC running linux, plus the windows and linux versions of Steam. I won't need a steambox to take advantage of it. It will work equally well with desktop pc's, running windows or linux, hopefully.
I also see this as a move to take direction and development of the pc gaming ecosystem out of Microsoft's sole control. Like when MS releases a new DirectX, and everyone now has to jump on it and adopt it, or having to tie in a lot of stuff to DotNet. There will be the possibility of other pressures on the evolution of PC gaming apart from Microsoft, and I think that might be the main benefit in the end.